This invention relates to blends of phosphors that convert electromagnetic radiation in the near-ultraviolet (xe2x80x9cnear-UVxe2x80x9d)-to-blue wavelength range to white light. More particularly, this invention relates to phosphor blends that convert electromagnetic radiation emitted by near-UV/blue light-emitting devices to white light. This invention also relates to light sources using such phosphor blends.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called xe2x80x9cactivatorsxe2x80x9d) have been added to convert them into efficient fluorescent materials. With the right combination of activators and host inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (xe2x80x9cUVxe2x80x9d) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or x rays (for example, scintillators in x-ray detection systems).
The efficiency of a lighting device that uses a phosphor increases as the difference between the wavelength of the exciting radiation and that of the emitted radiation narrows. Therefore, in the quest for improving efficiency of white light sources, effort has been dedicated to finding a source of stimulating radiation that has wavelengths longer than that of UV radiation and phosphors that respond to those wavelengths. Recent advances, in light-emitting diode (xe2x80x9cLEDxe2x80x9d) technology have brought efficient LEDs emitting in the near UV-to-blue range. The term xe2x80x9cLEDsxe2x80x9d as used herein also includes laser diodes. The term xe2x80x9cnear UVxe2x80x9d as used herein means UV radiation having wavelengths in the range from about 315 nm to about 410 nm. These LEDs emitting radiation in the near UV-to-blue range will be hereinafter called xe2x80x9cUV/blue LEDs.xe2x80x9d As used herein, a UV/blue LED may emit radiation having wavelengths in the near UV range, in the blue light range, or in a broad range from near UV to blue. It would be an advance to the technology of lighting to provide a range of phosphors that can be stimulated by the radiation emitted from these UV/blue LEDs radiation sources to allow for a flexibility in the use of phosphors for generating various color LEDs. Such phosphors when combined with the emission from the UV/blue LEDs can provide efficient and long lasting lighting devices that consume little power.
Many near UV/blue LEDs based on combinations of nitrides of indium, aluminum, and gallium have recently appeared. For example, U.S. Pat. No. 5,777,350 discloses LEDs comprising multiple layers of InGa and p- and n-type AlGaN, which emit in the wavelength range from about 380 nm to about 420 nm. Doping in the active layer of an LED can shift the peak emission from near UV to blue light wavelengths. A LED of the InGaN type emitting in the blue light wavelengths was combined with a coating of a yellow light-emitting yttrium aluminum garnet phosphor activated with cerium (xe2x80x9cYAG:Cexe2x80x9d) to produce white light and is disclosed in U.S. Pat. No. 5,998,925. Similarly, U.S. Pat. No. 6,066,861 discloses an yttrium aluminum garnet phosphor activated with terbium and/or cerium in which yttrium may be substituted with Ca and/or Sr, aluminum with Ga and/or Si, and oxygen with S, to be used as a component of a wavelength conversion layer for a blue light-emitting LED. YAG:Ce and its variations emit a broad-spectrum yellow light. Although a substantial portion of the need for white light devices may be filled by LED-based devices, the ability to combine a UV/blue LED with a phosphor has been limited because yttrium aluminum garnet phosphor and minor variations thereof have been the only known yellow light-emitting phosphors that are excitable by radiation in the blue range. This limitation has restricted, to some extent, the ability flexibly to design light sources having different color temperatures and achieving a high color rendering index (xe2x80x9cCRIxe2x80x9d).
Therefore, there is a need to provide phosphor compositions that are excitable in the near UV-to-blue range and emit in the visible range such that they may be used flexibly to design light sources having tunable properties, such as color temperature and CRI.
The present invention provides phosphor blends that are excitable by electromagnetic radiation having wavelengths in the near UV-to-blue range (from about 315 nm to about 480 nm) to emit efficiently a visible light in a range of wavelengths from about 490 nm to about 770 nm. A phosphor blend of the present invention comprises a mixture of at least two phosphors selected from the group consisting of (a) Sr2P2O7:Eu2+,Mn2+ (hereinafter called xe2x80x9cSr pyrophosphatexe2x80x9d); (b) (Ca,Sr,Ba)5(PO4)3(F,Cl,OH):Eu2+,Mn2+ (hereinafter called xe2x80x9cCa halophosphatexe2x80x9d); (c) 3.5MgO.0.5MgF2.GeO2:Mn4+ (hereinafter called xe2x80x9cMFGxe2x80x9d); (d) Sr4Al14O25:Eu2+ (hereinafter called xe2x80x9cSAExe2x80x9d); (e) (Sr,Ba,Ca)5(PO4)3(Cl,OH):Eu2+ (hereinafter called xe2x80x9cSECAxe2x80x9d); (f) an europium-activated aluminate phosphor selected from the group consisting of (Ba,Ca,Sr)2MgAl16O27:Eu2+, (Ba,Ca,Sr)MgAl10O17:Eu2+, and (Ba,Ca,Sr)Mg3Al14O25:Eu2+ (hereinafter called xe2x80x9cBAMxe2x80x9d); and (g) an europium and manganese co-activated aluminate phosphor selected from the group consisting of (Ba,Ca,Sr)2MgAl16O27:Eu2+,Mn2+, (Ba,Ca,Sr)MgAl10O17:Eu2+,Mn2+, and (Ba,Ca,Sr)Mg3Al14O25:Eu2+,Mn2+ (hereinafter called xe2x80x9cBaMnxe2x80x9d). By mixing appropriate proportions of these phosphors, composites of emission spectra may be created that provide a wide range of colors in the visible spectrum. Especially, white light sources having different color temperatures (xe2x80x9cCTxe2x80x9d) and CRIs may be created from a combination of near UV-to-blue LEDs and phosphor blends of the present invention.
Other aspects, advantages, and salient features of the present invention will become apparent from a perusal of the following detailed description, which, when taken in conjunction with the accompanying figures, discloses embodiments of the present invention.